Certain industrial processes require the use of speed reduction gear trains to reduce the customary speed of electrical motors to a few tens of rpm or less. The output shaft gear of such a gear train is physically quite large, perhaps several feet in diameter. Such a gear train and the housing that typically holds it will be referred to as a gearbox. An input shaft provides high speed low torque power.
The housing provides support for the bearings on which the gears' shafts turn, and also protects the gears from damage and holds a gear lubricant such as gear grease. A typical housing comprises a case and a cover. In one preferred design the gear train includes a pair of bevel gears for allowing an output shaft whose axis is at right angles to the input shaft.
The gear train has an output gear perhaps several feet in diameter that the output shaft carries. The torque that the output gear provides in these applications is often extremely large. For example, such a gear train may be used to slowly rotate a large industrial component holding materials with combined total weight of many tons. The large output torque of such a gear train produces on the teeth of both the output gear and the pinion gear driving the output gear, contact stresses that are extremely large as well.
Most gears are manufactured with teeth having cycloidal profiles, which provide for rolling line contact between meshing gears. Rolling contact avoids sliding between meshing teeth surfaces so little frictional wear occurs. Line contact (as opposed to point contact) reduces the Hertzian stress (force per unit contact area) on these teeth and further increases their life.
High precision machining now available for the teeth on such gears provides the opportunity for reliably creating during operation, continuous line contact between the individual teeth of the output gear and the meshing pinion gear. High precision bearings for the gear shafts further enhance the opportunity for continuous precision meshing of output gear teeth.
Unfortunately, output gear teeth continue to fail, often showing damage associated with excessive Hertzian stress. Such failure is costly, in that these gearboxes are very expensive. Further, it requires the entire installation to cease operation during gearbox or gear replacement. Since these gearboxes are often specially designed for a particular installation, replacement may take a long time. Alternatively, an installation may keep a spare gearbox available, reducing downtime but adding overhead costs to the operation of the installation.
Accordingly, the current state of affairs for installations using these large, high-torque gearboxes is unsatisfactory.